NAME

SYNOPSIS

DESCRIPTION

This document attempts to begin to describe some of the considerations involved in patching and maintaining perl.

This document is still under construction, and still subject to significant changes. Still, I hope parts of it will be useful, so I'm releasing it even though it's not done.

For the most part, it's a collection of anecdotal information that already assumes some familiarity with the Perl sources. I really need an introductory section that describes the organization of the sources and all the various auxiliary files that are part of the distribution.

Where Do I Get Perl Sources and Related Material?

The Comprehensive Perl Archive Network (or CPAN) is the place to go. There are many mirrors, but the easiest thing to use is probably http://www.perl.com/CPAN/README.html , which automatically points you to a mirror site "close" to you.

Perl5-porters mailing list

The mailing list perl5-porters@perl.org is the main group working with the development of perl. If you're interested in all the latest developments, you should definitely subscribe. The list is high volume, but generally has a fairly low noise level.

Subscribe by sending the message (in the body of your letter)

subscribe perl5-porters

to perl5-porters-request@perl.org .

Archives of the list are held at:

http://www.rosat.mpe-garching.mpg.de/mailing-lists/perl-porters/

How are Perl Releases Numbered?

Perl version numbers are floating point numbers, such as 5.004. (Observations about the imprecision of floating point numbers for representing reality probably have more relevance than you might imagine :-) The major version number is 5 and the '004' is the patchlevel. (Questions such as whether or not '004' is really a minor version number can safely be ignored.:)

The version number is available as the magic variable $], and can be used in comparisons, e.g.

print "You've got an old perl\n" if $] < 5.002;

You can also require particular version (or later) with

use 5.002;

At some point in the future, we may need to decide what to call the next big revision. In the .package file used by metaconfig to generate Configure, there are two variables that might be relevant: $baserev=5.0 and $package=perl5. At various times, I have suggested we might change them to $baserev=5.1 and $package=perl5.1 if want to signify a fairly major update. Or, we might want to jump to perl6. Let's worry about that problem when we get there.

Subversions

In addition, there may be "developer" sub-versions available. These are not official releases. They may contain unstable experimental features, and are subject to rapid change. Such developer sub-versions are numbered with sub-version numbers. For example, version 5.003_04 is the 4'th developer version built on top of 5.003. It might include the _01, _02, and _03 changes, but it also might not. Sub-versions are allowed to be subversive. (But see the next section for recent changes.)

These sub-versions can also be used as floating point numbers, so you can do things such as

print "You've got an unstable perl\n" if $] == 5.00303;

You can also require particular version (or later) with

use 5.003_03; # the "_" is optional

Sub-versions produced by the members of perl5-porters are usually available on CPAN in the src/5.0/unsupported directory.

Maintenance and Development Subversions

As an experiment, starting with version 5.004, subversions _01 through _49 will be reserved for bug-fix maintenance releases, and subversions _50 through _99 will be available for unstable development versions.

The separate bug-fix track is being established to allow us an easy way to distribute important bug fixes without waiting for the developers to untangle all the other problems in the current developer's release.

Trial releases of bug-fix maintenance releases are announced on perl5-porters. Trial releases use the new subversion number (to avoid testers installing it over the previous release) and include a 'local patch' entry in patchlevel.h.

Watch for announcements of maintenance subversions in comp.lang.perl.announce.

Why such a complicated scheme?

Two reasons, really. At least.

First, we need some way to identify and release collections of patches that are known to have new features that need testing and exploration. The subversion scheme does that nicely while fitting into the use 5.004; mold.

Second, since most of the folks who help maintain perl do so on a free-time voluntary basis, perl development does not proceed at a precise pace, though it always seems to be moving ahead quickly. We needed some way to pass around the "patch pumpkin" to allow different people chances to work on different aspects of the distribution without getting in each other's way. It wouldn't be constructive to have multiple people working on incompatible implementations of the same idea. Instead what was needed was some kind of "baton" or "token" to pass around so everyone knew whose turn was next.

Why is it called the patch pumpkin?

Chip Salzenberg gets credit for that, with a nod to his cow orker, David Croy. We had passed around various names (baton, token, hot potato) but none caught on. Then, Chip asked:

[begin quote]

Who has the patch pumpkin?

To explain: David Croy once told me once that at a previous job, there was one tape drive and multiple systems that used it for backups. But instead of some high-tech exclusion software, they used a low-tech method to prevent multiple simultaneous backups: a stuffed pumpkin. No one was allowed to make backups unless they had the "backup pumpkin".

[end quote]

The name has stuck.

Philosophical Issues in Patching Perl

There are no absolute rules, but there are some general guidelines I have tried to follow as I apply patches to the perl sources. (This section is still under construction.)

Solve problems as generally as possible

Never implement a specific restricted solution to a problem when you can solve the same problem in a more general, flexible way.

For example, for dynamic loading to work on some SVR4 systems, we had to build a shared libperl.so library. In order to build "FAT" binaries on NeXT 4.0 systems, we had to build a special libperl library. Rather than continuing to build a contorted nest of special cases, I generalized the process of building libperl so that NeXT and SVR4 users could still get their work done, but others could build a shared libperl if they wanted to as well.

Seek consensus on major changes

If you are making big changes, don't do it in secret. Discuss the ideas in advance on perl5-porters.

Keep the documentation up-to-date

If your changes may affect how users use perl, then check to be sure that the documentation is in sync with your changes. Be sure to check all the files pod/*.pod and also the INSTALL document.

Consider writing the appropriate documentation first and then implementing your change to correspond to the documentation.

Avoid machine-specific #ifdef's

To the extent reasonable, try to avoid machine-specific #ifdef's in the sources. Instead, use feature-specific #ifdef's. The reason is that the machine-specific #ifdef's may not be valid across major releases of the operating system. Further, the feature-specific tests may help out folks on another platform who have the same problem.

Allow for lots of testing

We should never release a main version without testing it as a subversion first.

Test popular applications and modules.

We should never release a main version without testing whether or not it breaks various popular modules and applications. A partial list of such things would include majordomo, metaconfig, apache, Tk, CGI, libnet, and libwww, to name just a few. Of course it's quite possible that some of those things will be just plain broken and need to be fixed, but, in general, we ought to try to avoid breaking widely-installed things.

Automate generation of derivative files

The embed.h, keywords.h, opcode.h, and perltoc.pod files are all automatically generated by perl scripts. In general, don't patch these directly; patch the data files instead.

Configure and config_h.SH are also automatically generated by metaconfig. In general, you should patch the metaconfig units instead of patching these files directly. However, very minor changes to Configure may be made in between major sync-ups with the metaconfig units, which tends to be complicated operations. But be careful, this can quickly spiral out of control. Running metaconfig is not really hard.

How to Make a Distribution

There really ought to be a 'make dist' target, but there isn't. The 'dist' suite of tools also contains a number of tools that I haven't learned how to use yet. Some of them may make this all a bit easier.

Here are the steps I go through to prepare a patch & distribution.

Lots of it could doubtless be automated but isn't. The Porting/makerel (make release) perl script does now help automate some parts of it.

Announce your intentions

First, you should volunteer out loud to take the patch pumpkin. It's generally counter-productive to have multiple people working in secret on the same thing.

At the same time, announce what you plan to do with the patch pumpkin, to allow folks a chance to object or suggest alternatives, or do it for you. Naturally, the patch pumpkin holder ought to incorporate various bug fixes and documentation improvements that are posted while he or she has the pumpkin, but there might also be larger issues at stake.

One of the precepts of the subversion idea is that we shouldn't give the patch pumpkin to anyone unless we have some idea what he or she is going to do with it.

refresh pod/perltoc.pod

Presumably, you have done a full make in your working source directory. Before you make spotless (if you do), and if you have changed any documentation in any module or pod file, change to the pod directory and run make toc.

run installhtml to check the validity of the pod files

update patchlevel.h

Don't be shy about using the subversion number, even for a relatively modest patch. We've never even come close to using all 99 subversions, and it's better to have a distinctive number for your patch. If you need feedback on your patch, go ahead and issue it and promise to incorporate that feedback quickly (e.g. within 1 week) and send out a second patch.

run metaconfig

If you need to make changes to Configure or config_h.SH, it may be best to change the appropriate metaconfig units instead, and regenerate Configure.

metaconfig -m

will regenerate Configure and config_h.SH. More information on obtaining and running metaconfig is in the U/README file that comes with Perl's metaconfig units. Perl's metaconfig units should be available the same place you found this file. On CPAN, look under my directory authors/id/ANDYD/ for a file such as 5.003_07-02.U.tar.gz. That file should be unpacked in your main perl source directory. It contains the files needed to run metaconfig to reproduce Perl's Configure script. (Those units are for 5.003_07. There have been changes since then; please contact me if you want more recent versions, and I will try to point you in the right direction.)

Alternatively, do consider if the *ish.h files might be a better place for your changes.

MANIFEST

Make sure the MANIFEST is up-to-date. You can use dist's manicheck program for this. You can also use

perl -w -MExtUtils::Manifest=fullcheck -e fullcheck

Both commands will also list extra files in the directory that are not listed in MANIFEST.

The MANIFEST is normally sorted, with one exception. Perl includes both a Configure script and a configure script. The configure script is a front-end to the main Configure, but is there to aid folks who use autoconf-generated configure files for other software. The problem is that Configure and configure are the same on case-insensitive file systems, so I deliberately put configure first in the MANIFEST so that the extraction of Configure will overwrite configure and leave you with the correct script. (The configure script must also have write permission for this to work, so it's the only file in the distribution I normally have with write permission.)

If you are using metaconfig to regenerate Configure, then you should note that metaconfig actually uses MANIFEST.new, so you want to be sure MANIFEST.new is up-to-date too. I haven't found the MANIFEST/MANIFEST.new distinction particularly useful, but that's probably because I still haven't learned how to use the full suite of tools in the dist distribution.

Check permissions

All the tests in the t/ directory ought to be executable. The main makefile used to do a 'chmod t/*/*.t', but that resulted in a self-modifying distribution--something some users would strongly prefer to avoid. Probably, the t/TEST script should check for this and do the chmod if needed, but it doesn't currently.

Update Porting/config.sh and Porting/config_H

[XXX This section needs revision. We're currently working on easing the task of keeping the vms, win32, and plan9 config.sh info up-to-date. The plan is to use keep up-to-date 'canned' config.sh files in the appropriate subdirectories and then generate 'canned' config.h files for vms, win32, etc. from the generic config.sh file. This is to ease maintenance. When Configure gets updated, the parts sometimes get scrambled around, and the changes in config_H can sometimes be very hard to follow. config.sh, on the other hand, can safely be sorted, so it's easy to track (typically very small) changes to config.sh and then propoagate them to a canned 'config.h' by any number of means, including a perl script in win32/ or carrying config.sh and config_h.SH to a Unix system and running sh config_h.SH.) XXX]

The Porting/config.sh and Porting/config_H files are provided to help those folks who can't run Configure. It is important to keep them up-to-date. If you have changed config_h.SH, those changes must be reflected in config_H as well. (The name config_H was chosen to distinguish the file from config.h even on case-insensitive file systems.) Simply edit the existing config_H file; keep the first few explanatory lines and then copy your new config.h below.

It may also be necessary to update vms/config.vms and plan9/config.plan9, though you should be quite careful in doing so if you are not familiar with those systems. You might want to issue your patch with a promise to quickly issue a follow-up that handles those directories.

make run_byacc

If you have byacc-1.8.2 (available from CPAN), and if there have been changes to perly.y, you can regenerate the perly.c file. The run_byacc makefile target does this by running byacc and then applying some patches so that byacc dynamically allocates space, rather than having fixed limits. This patch is handled by the perly.fixer script. Depending on the nature of the changes to perly.y, you may or may not have to hand-edit the patch to apply correctly. If you do, you should include the edited patch in the new distribution. If you have byacc-1.9, the patch won't apply cleanly. Changes to the printf output statements mean the patch won't apply cleanly. Long ago I started to fix perly.fixer to detect this, but I never completed the task.

This only happens when you add or remove a token type. I suppose this could be automated, but it doesn't happen very often nowadays.

Larry

make regen_headers

The embed.h, keywords.h, and opcode.h files are all automatically generated by perl scripts. Since the user isn't guaranteed to have a working perl, we can't require the user to generate them. Hence you have to, if you're making a distribution.

I used to include rules like the following in the makefile:

# The following three header files are generated automatically
# The correct versions should be already supplied with the perl kit,
# in case you don't have perl or 'sh' available.
# The - is to ignore error return codes in case you have the source
# installed read-only or you don't have perl yet.
keywords.h: keywords.pl
@echo "Don't worry if this fails."
- perl keywords.pl

However, I got lots of mail consisting of people worrying because the command failed. I eventually decided that I would save myself time and effort by manually running make regen_headers myself rather than answering all the questions and complaints about the failing command.

global.sym, interp.sym and perlio.sym

Make sure these files are up-to-date. Read the comments in these files and in perl_exp.SH to see what to do.

Binary compatibility

If you do change global.sym or interp.sym, think carefully about what you are doing. To the extent reasonable, we'd like to maintain souce and binary compatibility with older releases of perl. That way, extensions built under one version of perl will continue to work with new versions of perl.

Of course, some incompatible changes may well be necessary. I'm just suggesting that we not make any such changes without thinking carefully about them first. If possible, we should provide backwards-compatibility stubs. There's a lot of XS code out there. Let's not force people to keep changing it.

Changes

Be sure to update the Changes file. Try to include both an overall summary as well as detailed descriptions of the changes. Your audience will include other developers and users, so describe user-visible changes (if any) in terms they will understand, not in code like "initialize foo variable in bar function".

There are differing opinions on whether the detailed descriptions ought to go in the Changes file or whether they ought to be available separately in the patch file (or both). There is no disagreement that detailed descriptions ought to be easily available somewhere.

Todo

The Todo file contains a roughly-catgorized unordered list of aspects of Perl that could use enhancement, features that could be added, areas that could be cleaned up, and so on. During your term as pumpkin-holder, you will probably address some of these issues, and perhaps identify others which, while you decide not to address them this time around, may be tackled in the future. Update the file reflect the situation as it stands when you hand over the pumpkin.

You might like, early in your pumpkin-holding career, to see if you can find champions for partiticular issues on the to-do list: an issue owned is an issue more likely to be resolved.

There are also some more porting-specific Todo items later in this file.

OS/2-specific updates

In the os2 directory is diff.configure, a set of OS/2-specific diffs against Configure. If you make changes to Configure, you may want to consider regenerating this diff file to save trouble for the OS/2 maintainer.

You can also consider the OS/2 diffs as reminders of portability things that need to be fixed in Configure.

VMS-specific updates

If you have changed perly.y, then you may want to update vms/perly_{h,c}.vms by running perl vms/vms_yfix.pl.

The Perl version number appears in several places under vms. It is courteous to update these versions. For example, if you are making 5.004_42, replace "5.00441" with "5.00442".

Making the new distribution

Suppose, for example, that you want to make version 5.004_08. Then you can do something like the following

These steps, with extra checks, are automated by the Porting/makerel script.

Making a new patch

I find the makepatch utility quite handy for making patches. You can obtain it from any CPAN archive under http://www.perl.com/CPAN/authors/Johan_Vromans/ . There are a couple of differences between my version and the standard one. I have mine do a

# Print a reassuring "End of Patch" note so people won't
# wonder if their mailer truncated patches.
print "\n\nEnd of Patch.\n";

at the end. That's because I used to get questions from people asking if their mail was truncated.

It also writes Index: lines which include the new directory prefix (change Index: print, approx line 294 or 310 depending on the version, to read: print PATCH ("Index: $newdir$new\n");). That helps patches work with more POSIX conformant patch programs.

Here's how I generate a new patch. I'll use the hypothetical 5.004_07 to 5.004_08 patch as an example.

Makepatch will automatically generate appropriate rm commands to remove deleted files. Unfortunately, it will not correctly set permissions for newly created files, so you may have to do so manually. For example, patch 5.003_04 created a new test t/op/gv.t which needs to be executable, so at the top of the patch, I inserted the following lines:

# Make a new test
touch t/op/gv.t
chmod +x t/opt/gv.t

Now, of course, my patch is now wrong because makepatch didn't know I was going to do that command, and it patched against /dev/null.

So, what I do is sort out all such shell commands that need to be in the patch (including possible mv-ing of files, if needed) and put that in the shell commands at the top of the patch. Next, I delete all the patch parts of perl5.004_08.pat, leaving just the shell commands. Then, I do the following:

More testing

Again, it's obvious, but you should test your new version as widely as you can. You can be sure you'll hear about it quickly if your version doesn't work on both ANSI and pre-ANSI compilers, and on common systems such as SunOS 4.1.[34], Solaris, and Linux.

If your changes include conditional code, try to test the different branches as thoroughly as you can. For example, if your system supports dynamic loading, you can also test static loading with

sh Configure -Uusedl

You can also hand-tweak your config.h to try out different #ifdef branches.

Common Gotcha's

#elif

The '#elif' preprocessor directive is not understood on all systems. Specifically, I know that Pyramids don't understand it. Thus instead of the simple

Incidentally, whitespace between the leading '#' and the preprocessor command is not guaranteed, but is very portable and you may use it freely. I think it makes things a bit more readable, especially once things get rather deeply nested. I also think that things should almost never get too deeply nested, so it ought to be a moot point :-)

Probably Prefer POSIX

It's often the case that you'll need to choose whether to do something the BSD-ish way or the POSIX-ish way. It's usually not a big problem when the two systems use different names for similar functions, such as memcmp() and bcmp(). The perl.h header file handles these by appropriate #defines, selecting the POSIX mem*() functions if available, but falling back on the b*() functions, if need be.

More serious is the case where some brilliant person decided to use the same function name but give it a different meaning or calling sequence :-). getpgrp() and setpgrp() come to mind. These are a real problem on systems that aim for conformance to one standard (e.g. POSIX), but still try to support the other way of doing things (e.g. BSD). My general advice (still not really implemented in the source) is to do something like the following. Suppose there are two alternative versions, fooPOSIX() and fooBSD().

#ifndef MISSING_NEATO_FEATURE
/* Not missing it, so we must have it, so use it */
#else
/* Are missing it, so fall back on something else. */
#endif

Of course for this toy example, there's not much difference. But when the #ifdef's start spanning a couple of screen fulls, and the #else's are marked something like

#else /* !MISSING_NEATO_FEATURE */

I find it easy to get lost.

Providing Missing Functions -- Problem

Not all systems have all the neat functions you might want or need, so you might decide to be helpful and provide an emulation. This is sound in theory and very kind of you, but please be careful about what you name the function. Let me use the pause() function as an illustration.

Perl5.003 has the following in perl.h

#ifndef HAS_PAUSE
#define pause() sleep((32767<<16)+32767)
#endif

Configure sets HAS_PAUSE if the system has the pause() function, so this #define only kicks in if the pause() function is missing. Nice idea, right?

Unfortunately, some systems apparently have a prototype for pause() in unistd.h, but don't actually have the function in the library. (Or maybe they do have it in a library we're not using.)

/* Some unistd.h's give a prototype for pause() even though
HAS_PAUSE ends up undefined. This causes the #define
below to be rejected by the compiler. Sigh.
*/
#ifdef HAS_PAUSE
# define Pause pause
#else
# define Pause() sleep((32767<<16)+32767)
#endif

This works.

The curious reader may wonder why I didn't do the following in util.c instead:

#ifndef HAS_PAUSE
void pause()
{
sleep((32767<<16)+32767);
}
#endif

That is, since the function is missing, just provide it. Then things would probably be been alright, it would seem.

Well, almost. It could be made to work. The problem arises from the conflicting needs of dynamic loading and namespace protection.

For dynamic loading to work on AIX (and VMS) we need to provide a list of symbols to be exported. This is done by the script perl_exp.SH, which reads global.sym and interp.sym. Thus, the pause symbol would have to be added to global.sym So far, so good.

On the other hand, one of the goals of Perl5 is to make it easy to either extend or embed perl and link it with other libraries. This means we have to be careful to keep the visible namespace "clean". That is, we don't want perl's global variables to conflict with those in the other application library. Although this work is still in progress, the way it is currently done is via the embed.h file. This file is built from the global.sym and interp.sym files, since those files already list the globally visible symbols. If we had added pause to global.sym, then embed.h would contain the line

#define pause Perl_pause

and calls to pause in the perl sources would now point to Perl_pause. Now, when ld is run to build the perl executable, it will go looking for perl_pause, which probably won't exist in any of the standard libraries. Thus the build of perl will fail.

Those systems where HAS_PAUSE is not defined would be ok, however, since they would get a Perl_pause function in util.c. The rest of the world would be in trouble.

And yes, this scenario has happened. On SCO, the function chsize is available. (I think it's in -lx, the Xenix compatibility library.) Since the perl4 days (and possibly before), Perl has included a chsize function that gets called something akin to

#ifndef HAS_CHSIZE
I32 chsize(fd, length)
/* . . . */
#endif

When 5.003 added

#define chsize Perl_chsize

to embed.h, the compile started failing on SCO systems.

The "fix" is to give the function a different name. The one implemented in 5.003_05 isn't optimal, but here's what was done:

Undef and then re-define my_chsize from Perl_my_chsize to
just plain chsize if this system HAS_CHSIZE. This probably only
applies to SCO. This shows the perils of having internal
functions with the same name as external library functions :-).

Now, we can safely put my_chsize in global.sym, export it, and hide it with embed.h.

To be consistent with what I did for pause, I probably should have called the new function Chsize, rather than my_chsize. However, the perl sources are quite inconsistent on this (Consider New, Mymalloc, and Myremalloc, to name just a few.)

There is a problem with this fix, however, in that Perl_chsize was available as a libperl.a library function in 5.003, but it isn't available any more (as of 5.003_07). This means that we've broken binary compatibility. This is not good.

Providing missing functions -- some ideas

We currently don't have a standard way of handling such missing function names. Right now, I'm effectively thinking aloud about a solution. Some day, I'll try to formally propose a solution.

Part of the problem is that we want to have some functions listed as exported but not have their names mangled by embed.h or possibly conflict with names in standard system headers. We actually already have such a list at the end of perl_exp.SH (though that list is out-of-date):

This still needs much thought, but I'm inclined to think that one possible solution is to prefix all such functions with perl_ in the source and list them along with the other perl_* functions in perl_exp.SH.

Alternatively, we could just always use chsize everywhere and move chsize from global.sym to the end of perl_exp.SH. That would probably be fine as long as our chsize function agreed with all the chsize function prototypes in the various systems we'll be using. As long as the prototypes in actual use don't vary that much, this is probably a good alternative. (As a counter-example, note how Configure and perl have to go through hoops to find and use get Malloc_t and Free_t for malloc and free.)

At the moment, this latter option is what I tend to prefer.

All the world's a VAX

Sorry, showing my age:-). Still, all the world is not BSD 4.[34], SVR4, or POSIX. Be aware that SVR3-derived systems are still quite common (do you have any idea how many systems run SCO?) If you don't have a bunch of v7 manuals handy, the metaconfig units (by default installed in /usr/local/lib/dist/U) are a good resource to look at for portability.

Miscellaneous Topics

Autoconf

Why does perl use a metaconfig-generated Configure script instead of an autoconf-generated configure script?

Metaconfig and autoconf are two tools with very similar purposes. Metaconfig is actually the older of the two, and was originally written by Larry Wall, while autoconf is probably now used in a wider variety of packages. The autoconf info file discusses the history of autoconf and how it came to be. The curious reader is referred there for further information.

Overall, both tools are quite good, I think, and the choice of which one to use could be argued either way. In March, 1994, when I was just starting to work on Configure support for Perl5, I considered both autoconf and metaconfig, and eventually decided to use metaconfig for the following reasons:

Compatibility with Perl4

Perl4 used metaconfig, so many of the #ifdef's were already set up for metaconfig. Of course metaconfig had evolved some since Perl4's days, but not so much that it posed any serious problems.

Metaconfig worked for me

My system at the time was Interactive 2.2, a SVR3.2/386 derivative that also had some POSIX support. Metaconfig-generated Configure scripts worked fine for me on that system. On the other hand, autoconf-generated scripts usually didn't. (They did come quite close, though, in some cases.) At the time, I actually fetched a large number of GNU packages and checked. Not a single one configured and compiled correctly out-of-the-box with the system's cc compiler.

Configure can be interactive

With both autoconf and metaconfig, if the script works, everything is fine. However, one of my main problems with autoconf-generated scripts was that if it guessed wrong about something, it could be very hard to go back and fix it. For example, autoconf always insisted on passing the -Xp flag to cc (to turn on POSIX behavior), even when that wasn't what I wanted or needed for that package. There was no way short of editing the configure script to turn this off. You couldn't just edit the resulting Makefile at the end because the -Xp flag influenced a number of other configure tests.

Metaconfig's Configure scripts, on the other hand, can be interactive. Thus if Configure is guessing things incorrectly, you can go back and fix them. This isn't as important now as it was when we were actively developing Configure support for new features such as dynamic loading, but it's still useful occasionally.

GPL

At the time, autoconf-generated scripts were covered under the GNU Public License, and hence weren't suitable for inclusion with Perl, which has a different licensing policy. (Autoconf's licensing has since changed.)

Modularity

Metaconfig builds up Configure from a collection of discrete pieces called "units". You can override the standard behavior by supplying your own unit. With autoconf, you have to patch the standard files instead. I find the metaconfig "unit" method easier to work with. Others may find metaconfig's units clumsy to work with.

@INC search order

By default, the list of perl library directories in @INC is the following:

$archlib
$privlib
$sitearch
$sitelib

Specifically, on my Solaris/x86 system, I run sh Configure -Dprefix=/opt/perl and I have the following directories:

That is, perl's directories come first, followed by the site-specific directories.

The site libraries come second to support the usage of extensions across perl versions. Read the relevant section in INSTALL for more information. If we ever make $sitearch version-specific, this topic could be revisited.

Why isn't there a directory to override Perl's library?

Mainly because no one's gotten around to making one. Note that "making one" involves changing perl.c, Configure, config_h.SH (and associated files, see above), and documenting it all in the INSTALL file.

Apparently, most folks who want to override one of the standard library files simply do it by overwriting the standard library files.

APPLLIB

In the perl.c sources, you'll find an undocumented APPLLIB_EXP variable, sort of like PRIVLIB_EXP and ARCHLIB_EXP (which are documented in config_h.SH). Here's what APPLLIB_EXP is for, from a mail message from Larry:

The main intent of APPLLIB_EXP is for folks who want to send out a
version of Perl embedded in their product. They would set the symbol
to be the name of the library containing the files needed to run or to
support their particular application. This works at the "override"
level to make sure they get their own versions of any library code that
they absolutely must have configuration control over.
As such, I don't see any conflict with a sysadmin using it for a
override-ish sort of thing, when installing a generic Perl. It should
probably have been named something to do with overriding though. Since
it's undocumented we could still change it... :-)

Given that it's already there, you can use it to override distribution modules. If you do

sh Configure -Dccflags='-DAPPLLIB_EXP=/my/override'

then perl.c will put /my/override ahead of ARCHLIB and PRIVLIB.

Shared libperl.so location

Why isn't the shared libperl.so installed in /usr/lib/ along with "all the other" shared libraries? Instead, it is installed in $archlib, which is typically something like

/usr/local/lib/perl5/archname/5.00404

and is architecture- and version-specific.

The basic reason why a shared libperl.so gets put in $archlib is so that you can have more than one version of perl on the system at the same time, and have each refer to its own libperl.so.

Three examples might help. All of these work now; none would work if you put libperl.so in /usr/lib.

Suppose you want to have both threaded and non-threaded perl versions around. Configure will name both perl libraries "libperl.so" (so that you can link to them with -lperl). The perl binaries tell them apart by having looking in the appropriate $archlib directories.

Suppose you have perl5.004_04 installed and you want to try to compile it again, perhaps with different options or after applying a patch. If you already have libperl.so installed in /usr/lib/, then it may be either difficult or impossible to get ld.so to find the new libperl.so that you're trying to build. If, instead, libperl.so is tucked away in $archlib, then you can always just change $archlib in the current perl you're trying to build so that ld.so won't find your old libperl.so. (The INSTALL file suggests you do this when building a debugging perl.)

The shared perl library is not a "well-behaved" shared library with proper major and minor version numbers, so you can't necessarily have perl5.004_04 and perl5.004_05 installed simultaneously. Suppose perl5.004_04 were to install /usr/lib/libperl.so.4.4, and perl5.004_05 were to install /usr/lib/libperl.so.4.5. Now, when you try to run perl5.004_04, ld.so might try to load libperl.so.4.5, since it has the right "major version" number. If this works at all, it almost certainly defeats the reason for keeping perl5.004_04 around. Worse, with development subversions, you certaily can't guarantee that libperl.so.4.4 and libperl.so.4.55 will be compatible.

Anyway, all this leads to quite obscure failures that are sure to drive casual users crazy. Even experienced users will get confused :-). Upon reflection, I'd say leave libperl.so in $archlib.

Upload Your Work to CPAN

You can upload your work to CPAN if you have a CPAN id. Check out http://www.perl.com/CPAN/modules/04pause.html for information on _PAUSE_, the Perl Author's Upload Server.

I typically upload both the patch file, e.g. perl5.004_08.pat.gz and the full tar file, e.g. perl5.004_08.tar.gz.

If you want your patch to appear in the src/5.0/unsupported directory on CPAN, send e-mail to the CPAN master librarian. (Check out http://www.perl.com/CPAN/CPAN.html ).

Help Save the World

You should definitely announce your patch on the perl5-porters list. You should also consider announcing your patch on comp.lang.perl.announce, though you should make it quite clear that a subversion is not a production release, and be prepared to deal with people who will not read your disclaimer.

Todo

Here, in no particular order, are some Configure and build-related items that merit consideration. This list isn't exhaustive, it's just what I came up with off the top of my head.

Good ideas waiting for round tuits

installprefix

I think we ought to support

Configure -Dinstallprefix=/blah/blah

Currently, we support -Dprefix=/blah/blah, but the changing the install location has to be handled by something like the config.over trick described in INSTALL. AFS users also are treated specially. We should probably duplicate the metaconfig prefix stuff for an install prefix.

Configure -Dsrc=/blah/blah

We should be able to emulate configure --srcdir. Tom Tromey tromey@creche.cygnus.com has submitted some patches to the dist-users mailing list along these lines. They have been folded back into the main distribution, but various parts of the perl Configure/build/install process still assume src='.'.

Hint file fixes

Various hint files work around Configure problems. We ought to fix Configure so that most of them aren't needed.

Hint file information

Some of the hint file information (particularly dynamic loading stuff) ought to be fed back into the main metaconfig distribution.

Catch GNU Libc "Stub" functions

Some functions (such as lchown()) are present in libc, but are unimplmented. That is, they always fail and set errno=ENOSYS.

Thomas Bushnell provided the following sample code and the explanation that follows:

/* System header to define __stub macros and hopefully few prototypes,
which can conflict with char FOO(); below. */
#include <assert.h>
/* Override any gcc2 internal prototype to avoid an error. */
/* We use char because int might match the return type of a gcc2
builtin and then its argument prototype would still apply. */
char FOO();
int main() {
/* The GNU C library defines this for functions which it implements
to always fail with ENOSYS. Some functions are actually named
something starting with __ and the normal name is an alias. */
#if defined (__stub_FOO) || defined (__stub___FOO)
choke me
#else
FOO();
#endif
; return 0; }

The choice of <assert.h> is essentially arbitrary. The GNU libc macros are found in <gnu/stubs.h>. You can include that file instead of <assert.h> (which itself includes <gnu/stubs.h>) if you test for its existence first. <assert.h> is assumed to exist on every system, which is why it's used here. Any GNU libc header file will include the stubs macros. If either __stub_NAME or __stub___NAME is defined, then the function doesn't actually exist. Tests using <assert.h> work on every system around.

The declaration of FOO is there to override builtin prototypes for ANSI C functions.

Probably good ideas waiting for round tuits

GNU configure --options

I've received sensible suggestions for --exec_prefix and other GNU configure --options. It's not always obvious exactly what is intended, but this merits investigation.

make clean

Currently, make clean isn't all that useful, though make realclean and make distclean are. This needs a bit of thought and documentation before it gets cleaned up.

Try gcc if cc fails

Currently, we just give up.

bypassing safe*alloc wrappers

On some systems, it may be safe to call the system malloc directly without going through the util.c safe* layers. (Such systems would accept free(0), for example.) This might be a time-saver for systems that already have a good malloc. (Recent Linux libc's apparently have a nice malloc that is well-tuned for the system.)

Vague possibilities

MacPerl

Get some of the Macintosh stuff folded back into the main distribution.

gconvert replacement

Maybe include a replacement function that doesn't lose data in rare cases of coercion between string and numerical values.

Improve makedepend

The current makedepend process is clunky and annoyingly slow, but it works for most folks. Alas, it assumes that there is a filename $firstmakefile that the make command will try to use before it uses Makefile. Such may not be the case for all make commands, particularly those on non-Unix systems.

Probably some variant of the BSD .depend file will be useful. We ought to check how other packages do this, if they do it at all. We could probably pre-generate the dependencies (with the exception of malloc.o, which could probably be determined at Makefile.SH extraction time.

GNU Makefile standard targets

GNU software generally has standardized Makefile targets. Unless we have good reason to do otherwise, I see no reason not to support them.